Method and system for precoding data

ABSTRACT

The present disclosure relates to channel state feedback in a communication system. The method includes obtaining a reference signal from an access point; deriving a rank indication, a codebook subset selection indication and a precoding matrix index based on the obtained reference signal; sending a first feedback message conveying the rank indication and the codebook subset selection indication, and sending a second feedback message conveying the precoding matrix index, to the access point; and receiving, from the access point, data precoded by a matrix derived based on the rank indication, the codebook subset selection indication and the precoding matrix index.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/157,597, filed on Jan. 17, 2014, which is a continuation of U.S.patent application Ser. No. 13/603,441, filed on Sep. 5, 2012. U.S.patent application Ser. No. 13/603,441 is a continuation ofInternational Patent Application No. PCT/CN2010/076383, filed on Aug.26, 2010. The afore-mentioned patent applications are herebyincorporated by reference in their entireties.

TECHNICAL FIELD

The present invention relates to communication technologies, and inparticular, to a method and system for precoding data.

BACKGROUND

In Multiple Input Multiple Output (MIMO) Orthogonal Frequency DivisionMultiplexing Access (OFDMA) cellular systems, closed-loop MIMO precodingis defined to enable high throughput downlink transmissions to fixed andnomadic user terminals. Feedback information is obtained by atransmitting device via a report of a precoding matrix index (PMI) froma receiving device. This PMI is used to represent a matrix in a codebook(a codebook is a set of matrices), and the PMI is used by thetransmitting device to derive a downlink precoder. While the concepts of“transmitting device” and “receiving device” are relevant to each other,in the scenarios involved in this application, the transmitting devicemay be in a form of a network entity, such as a base station, an evolvedNodeB (eNB) or an access point. The receiving device may be in a form ofa user equipment (UE), such as a mobile device, a cellular telephone, awireless enabled personal digital assistant (PDA), a wireless enabledlaptop computer, or other such devices. However, it is noted that thepresent application is not limited by these scenarios.

Simple codebook structures are used in the first generation of MIMOOFDMA systems, such that the PMI can be represented in a few bits. Smallcodebooks can be used to achieve a coarse quantization of a spatialchannel. For example, a 4-bit codebook is defined and a 4-bit PMI can bereported via a capacity-limited feedback channel called Physical uplinkcontrol channel (PUCCH). Precoding matrices are defined for eachpossible transmission rank, where the transmission rank determines thesize of the precoding matrix. The overall codebook is a set of matricesfor each rank. The feedback report thus includes a rank indication (RI)and a PMI in the codebook subset of the given rank. Since the rank ofthe propagation channel varies slowly in comparison with fast fadingover which the PMI is adapted, the rank indication is reported with alonger period than the PMI.

Since the RI remains valid during several reports of the PMI, it can beencoded with a better error protection code in order to ensure thatconsecutive PMI reports are not invalidated by one erroneous RI report.In general, this is naturally ensured by the fact that the rank can takevalues in a limited range, such as {1, 2, 3, 4} in LTE (Long TermEvolution) system Rel-8, due to the limitation to a maximum of 4antennas at the transmitting device and 4 antennas at the receivingdevice. Thus the RI can be represented by 2 bits. Since the PMI is ingeneral reported along with a channel quality indication (CQI) thatrepresents the channel quality, assuming that the transmitting deviceprecodes data with the reported PMI, the total size of the feedbackmessage that contains the PMI+CQI is larger than the 4 bits used torepresent just the PMI. For example, in LTE Rel-8, the message size willbe 8 to 11 bits with one to two CQIs, respectively. The PUCCH istransmitted in a fixed-size time-frequency resource with a fixedmodulation. The 2 bits of RI that are reported individually are betterprotected than the 8 or 11 bits of PMI and CQI that are jointlyreported, since the 2 bits of RI enjoy a lower error-correction encodingrate.

SUMMARY

According to a first aspect of the present invention, a method forprecoding data in a wireless communication system is provided. Themethod includes the following:

-   -   receiving, by a receiving device, a reference signal from a        transmitting device;    -   deriving, by the receiving device, a rank indication, a codebook        index n1 in a first codebook C1, codebook indices k1 and k2 in a        second codebook C2, a wideband channel quality indicator CQI1        and a subband channel quality indicator CQI2, based on the        received reference signal;    -   sending, by the receiving device, the rank indication, the        codebook index n1 in the first codebook C1, the codebook indices        k1 and k2 in the second codebook C2, the wideband channel        quality indicator CQI1 and the subband channel quality indicator        CQI2, to the transmitting device; and    -   receiving, by the receiving device, data precoded by a first        precoder from the transmitting device. The first precoder is        determined by the transmitting device based on the rank        indication and the indices n1, k1 and k2.

According to a second aspect of the invention, a receiving device isprovided. The receiving device includes the following:

-   -   a receiver, configured to receive a reference signal from a        transmitting device;    -   a processor, configured to derive a rank indication, a codebook        index n1 in a first codebook C1 and codebook indices k1 and k2        in a second codebook C2, a wideband channel quality indicator        CQI1 and a subband channel quality indicator CQI2 based on the        reference signal; and    -   a transmitter, configured to send the rank indication, the        codebook index n1 in the first codebook C1, the codebook indices        k1 and k2 in the second codebook C2, the wideband channel        quality indicator CQI1 and the subband channel quality indicator        CQI2 to the transmitting device.

Further, the receiver is configured to receive, from the transmittingdevice, data precoded by a first precoder. The first precoder isdetermined by the transmitting device based on the rank indication andthe indices n1, k1 and k2.

The above receiving device may be a user equipment (UE) such as a mobiledevice, a cellular telephone, a wireless enabled personal digitalassistant (PDA), or a wireless enabled laptop computer.

According to a third aspect of the invention, another method forprecoding data in a wireless communication system is provided. Themethod includes the following:

-   -   receiving, by a transmitting device, a rank indication, a        codebook index n1 in a first codebook C1, codebook indices k1        and k2 in a second codebook C2, a wideband channel quality        indicator CQI1 and a subband channel quality indicator CQI2 from        a receiving device;    -   determining, by the transmitting device, a first precoder based        on the rank indication and the indices n1, k1 and k2;    -   precoding, by the transmitting device, data to be sent to the        receiving device using the first precoder; and    -   sending, by the transmitting device, the precoded data to the        receiving device.

According to a fourth aspect of the invention, a transmitting device isprovided. The transmitting device includes the following:

-   -   a receiver, configured to receive a rank indication, a codebook        index n1 in a first codebook C1, codebook indices k1 and k2 in a        second codebook C2, a wideband channel quality indicator CQI1        and a subband channel quality indicator CQI2 from a receiving        device; a processor, configured to determine a first precoder        based on the rank indication and the indices n1, k1 and k2, and        precode data to be sent to the receiving device using the first        precoder; and    -   a transmitter, configured to send the precoded data to the        receiving device.

The above transmitting device may be a network entity such as basestation, an evolved Node B or an access point.

According a fifth aspect of the invention, a system for precoding datais provided. The system includes a transmitting device and a receivingdevice as detailed above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of a method for precoding data according to anembodiment of the present invention;

FIG. 2 is a flowchart of a method for precoding data according toanother embodiment of the invention;

FIG. 3 is a flowchart of a method for precoding data according to yetanother embodiment of the invention;

FIG. 4 is a simplified block diagram of a receiving device according toan embodiment of the invention;

FIG. 5 is a simplified block diagram of a transmitting device accordingto an embodiment of the invention;

FIG. 6 is a flowchart of a method for precoding data according to anembodiment of the present invention; and

FIG. 7 is a simplified block diagram of a system according to anembodiment of the invention.

DETAILED DESCRIPTION

Particularly, embodiments of the present invention relate to reportingmultiple quantized representation of channel state information of apropagation channel in a capacity-limited feedback channel using adouble codebook structure. A wireless communication system comprises atleast one transmitting device and at least one receiving device. Thetransmitting device may be in a form of a network entity, such as a basestation, an evolved NodeB (eNB) or an access point. The receiving devicemay be in a form of a user equipment (UE), such as a mobile device, acellular telephone, a wireless enabled personal digital assistant (PDA)a wireless enabled laptop computer, or other such devices. At least onereference signal to the receiving device and a feedback channel from thereceiving device allow the transmission from the transmitting deviceusing codebook-based precoding when the transmitting device is equippedwith multiple transmit antennas.

For high-throughput operation in MIMO OFDMA systems, such as the LTEsystem, IEEE 802.16e/m system or other such systems, a receiving devicemay be asked to feedback one PMI for each subband of the bandwidth ofthe system, since it offers a better granularity than a widebandprecoder and allows the transmitting device to adapt its precoder to thefrequency-selectivity of the wireless propagation channel, as well asproviding information for multiuser diversity scheduling. In order totake advantage of these properties while limiting the rate of feedbackin the channel to an acceptable level, a receiving device may be askedto report the PMIs for a set of selected subbands that do not span theentire bandwidth of operation. Additionally, in order to ensure a smoothoperation of the scheduler at the transmitting device, it is often alsorequired that the receiving device reports a wideband PMI along with thesubband PMIs of selected subbands, so that the transmitting device haschannel state information for scheduling that receiving device in any ofthe subbands for which a subband PMI is not reported. As pointed outabove, a CQI is reported in association with a PMI, so a wideband CQI isreported along the wideband PMI, while a subband CQI is reported foreach of the subband PMIs.

Enhancements of the feedback for closed-loop precoding beyond the simple4-bit codebook or antenna array sizes for larger transmission wouldrequire more complex codebook structures, as well as a larger codebooksize. A first type of enhanced codebook contains matrices that provide aquantization of the MIMO propagation channel between an array oftransmitting antennas and an array of receiving antennas, where thematrices take the form of:W=W_(MI1) ⁽¹⁾W_(MI2) ⁽²⁾  (1)

The first matrix W_(MI1) ⁽¹⁾ in equation (1) is an inner precoder and ithas a block diagonal structure of

$\begin{matrix}{W_{{MI}\; 1}^{(1)} = \begin{bmatrix}{\overset{\sim}{W}}_{{MI}\; 1}^{(1)} & 0 \\0 & {\overset{\sim}{W}}_{{MI}\; 1}^{(1)}\end{bmatrix}} & (2)\end{matrix}$

For rank 1, the precoder could be formed as

$\begin{matrix}{{W = {\begin{bmatrix}{\overset{\sim}{W}}_{{MI}\; 1}^{(1)} & 0 \\0 & {\overset{\sim}{W}}_{{MI}\; 1}^{(1)}\end{bmatrix}\begin{bmatrix}1 \\\alpha\end{bmatrix}}},{\alpha \in \left\{ {1,{- 1},j,{- j}} \right\}}} & (3)\end{matrix}$

The rank 2 case would follow similarly as

$\begin{matrix}{{W = {\begin{bmatrix}{\overset{\sim}{W}}_{{MI}\; 1}^{(1)} & 0 \\0 & {\overset{\sim}{W}}_{{MI}\; 1}^{(1)}\end{bmatrix}\begin{bmatrix}1 & 1 \\\alpha & {–\alpha}\end{bmatrix}}},{\alpha \in \left\{ {1,j} \right\}}} & (4)\end{matrix}$

The second matrix in equation (1) represents a co-phasing factor thatreflects short-term channel variations and can be made to adjust thestructure of the overall precoder W to match either a single-polarizedlinear uniform array (ULA) or a cross-polarized array at thetransmitting device.

The embodiments of the invention described herein may be used forvarious broadband wireless communication systems where a double codebookof precoding matrices is known by a transmitting device and a receivingdevice and the transmitting device is equipped with multiple antennas,some of which may have the same or different polarizations. And thecodebook is used for sending feedback information from the receivingdevice to the transmitting device via a capacity-limited feedbackchannel. A double codebook is composed of two codebooks, which are setsof matrices, and the combination of one matrix from a first codebook C1and one matrix from a second codebook C2 forms a precoder that can beused by the transmitting device to send data to a receiving device viamultiple transmit antennas.

In an embodiment, a receiving device, which can be a UE, is connected toa transmitting device, which can be an access point, for a duration muchlonger than a transmission time interval. During this duration, thereceiving device would send several feedback reports containing indicesin a double codebook to recommend one or several precoders to thetransmitting device.

In one embodiment, at least one message including a rank indication, afirst codebook index n1 in a first codebook C1, and two codebook indicesk1 and k2 in a second codebook C2, which is contained in at least onereport, is fed back from the receiving device to the transmittingdevice. The indices (n1, k1) represent a wideband precoder W1 in thedouble codebook, while the indices (n1, k2) represent a subband precoderW2 in the double codebook, based on the same matrix with index n1 in thecodebook C1. A wideband channel quality indicator (CQI1) is alsoreported in reference to the wideband precoder represented by (n1, k1),while a subband channel quality indicator (CQI2) is also reported inreference to the subband precoder represented by (n1, k2).

In some embodiments where the index k1 is not reported, it is assumed tobe pre-defined with a common assumption known by both the receivingdevice and the transmitting device.

In an embodiment, a method for precoding feedback in a wirelesscommunication system is disclosed. Referring to FIG. 1, the methodincludes:

S201: Obtaining, by a receiving device, a reference signal from atransmitting device;

S202: Deriving, by the receiving device, a rank indication, a firstcodebook index n1 in a first codebook C1, two codebook indices k1 and k2in a second codebook C2, a wideband channel quality indicator CQI1associated with a wideband precoding matrix represented by indices (n1,k1), and a subband channel quality indicator CQI2 associated with awideband precoding matrix represented by indices (n1, k2) from thereference signal;

S203: sending, by the receiving device, a feedback report conveying therank indication, the first codebook index n1, the two codebook indicesk1 and k2, the wideband channel quality indicator CQI1, and the subbandchannel quality indicator CQI2, to the transmitting device; and

S204: receiving, by the receiving device from the transmitting device,data precoded by a first precoder determined by the transmitting devicebased on the rank indication and the indices n1, k1 and k2.

In one instance, the first precoder is determined by the transmittingdevice based on the rank indication and the indices n1, k1 and k2.

The first precoder is determined based on the rank indication, a secondprecoder and a third precoder. The second precoder, which represents awideband precoder, is determined by the indices n1 and k1. The thirdprecoder, which represents a subband precoder, is determined by theindices n1 and k2.

Determining the second precoder based on the indices n1 and k1 includes:

The second precoder is determined as W=W_(n) ₁ ⁽¹⁾W_(k) ₁ ⁽²⁾. W_(n) ₁⁽¹⁾ represents the matrix determined by the index n1 in the codebook C1,and W_(k) ₁ ⁽²⁾ represents the matrix determined by the index k1 in thecodebook C2.

Determining the third precoder based on the indices n1 and k2 includes:

The third precoder is determined as W=W_(n) ₁ ⁽¹⁾W_(k) ₂ ⁽²⁾. W_(n) ₁⁽¹⁾ represents the matrix determined by the index n1 in the codebook C1,and W_(k) ₂ ⁽²⁾ represents the matrix determined by the index k2 in thecodebook C2.

Alternatively, determining the second precoder based on the indices n1and k1 includes:

The second precoder is determined as W=W_(k) ₁ ⁽²⁾W_(n) ₁ ⁽¹⁾. W_(k) ₁⁽²⁾ represents the matrix determined by the index k1 in the codebook C2,and W_(n) ₁ ⁽¹⁾ represents the matrix determined by the index n1 in thecodebook C1.

Determining the third precoder by the indices n1 and k2 at the includes:

The third precoder is determined as W=W_(k) ₂ ⁽²⁾W_(n) ₁ ⁽¹⁾. W_(k) ₂⁽²⁾ represents the matrix determined by the index k2 in the codebook C2,and W_(n) ₁ ⁽¹⁾ represents the matrix determined by the index n1 in thecodebook C1.

The method above allows for reducing the number of reported indices forrepresenting two precoders in a double codebook, and there is noperformance impact.

In one embodiment, the transmitting device may ask the receiving deviceto feed back in a series of messages as exemplified in FIG. 2.

S301: the receiving device sends a first message that contains the rankindication.

S302: the receiving device sends a second message that contains theindices n1 and k1 and the wideband channel quality indicator CQI1.

S303: the receiving device sends a third message that contains the indexk2 and the subband channel quality indicator CQI2.

S304: the receiving device receives, from the transmitting device, dataprecoded by a matrix derived based on the rank indication and theindices n1, k1 and k2.

In one embodiment, the transmitting device may ask the receiving deviceto feed back in a series of compact messages that can contain one matrixindex, as exemplified in FIG. 3.

S401: the receiving device sends a first message that contains the rankindication.

S402: the receiving device sends a second message that contains theindex n1 and the wideband channel quality indicator CQI1.

S403: the receiving device sends a third message that contains the indexk2 and the subband channel quality indicator CQI2. And the index k1 maybe implicitly known by both the transmitting device and the receivingdevice as a fixed matrix index in the codebook C2.

S404: the receiving device receives, from the transmitting device, dataprecoded by a matrix derived based on the rank indication and theindices n1, k1 and k2.

In one embodiment, a precoding matrix in the double codebook is derivedas W=W⁽²⁾W⁽¹⁾, where W⁽¹⁾ is a matrix from the codebook C1, and W⁽²⁾ isa matrix from the codebook C2. The wideband precoder W₁ is derived atthe receiving device as W₁=W_(k) ₁ ⁽²⁾W_(n) ₁ ⁽¹⁾, and the subbandprecoder W₂ is derived as W₂=W_(k) ₂ ⁽²⁾W_(n) ₁ ⁽¹⁾. A wideband CQI,denoted as CQI1, represents the channel quality assuming the widebandprecoding matrix is used at the transmitting device. A subband CQI,denoted as CQI2, represents the channel quality assuming the subbandprecoding matrix is used at the transmitting device.

In one embodiment, the index k1 is pre-defined at both the transmittingdevice and receiving device based on a fixed assumption, and the matrixwhich the index k1 represents may be the identity matrix, in which casethe wideband precoder is simply represented as W_(n) ₁ ⁽¹⁾.

In one embodiment, a precoding matrix in the double codebook is derivedas W=W⁽¹⁾W⁽²⁾, where W⁽¹⁾ is a matrix from the codebook C1, and W⁽²⁾ isa matrix from the codebook C2. The wideband precoder W₁ is derived atthe receiving device as W₁=W_(n) ₁ ⁽¹⁾W_(k) ₁ ⁽²⁾, and the subbandprecoder W₂ is derived as W₂=W_(n) ₁ ⁽¹⁾W_(k) ₂ ⁽²⁾. A wideband CQI,denoted as CQI1, represents the channel quality assuming the widebandprecoding matrix is used at the transmitting device. A subband CQI,denoted as CQI2, represents the channel quality assuming the subbandprecoding matrix is used at the transmitting device. In one embodimentthe index k1 is pre-defined at both the transmitting device andreceiving device based on a fixed assumption, and the matrix which theindex k1 represents may be the identity matrix or a block matrixcomposed of identity matrices of smaller dimension.

In one embodiment, a precoding matrix in the double codebook is derivedas W=W⁽¹⁾W⁽²⁾, where W⁽¹⁾ is a matrix from the codebook C1, and W⁽²⁾ isa matrix from the codebook C2. The wideband precoder W₁ is derived atthe receiving device as W₁=W_(n) ₁ ⁽¹⁾W_(k) ₁ ⁽²⁾, and the subbandprecoder W₂ is derived as W₂=W_(n) ₁ ⁽¹⁾W_(k) ₂ ⁽²⁾. A wideband CQI,denoted as CQI1, represents the channel quality assuming the widebandprecoding matrix is used at the transmitting device. A subband CQI,denoted as CQI2, represents the channel quality assuming the subbandprecoding matrix is used at the transmitting device.

In one embodiment the index k1 is reported based on a subset of thecodebook C2, and k1 may be reported with fewer bits than the index k2.For example, k2 is the index in the subset of the codebook C2.

In one embodiment, a precoding matrix in the double codebook is derivedas W=W⁽¹⁾W⁽²⁾, where R=W⁽¹⁾ is a normalized covariance matrix from thecodebook C1, and W⁽²⁾ is a matrix from the codebook C2. The widebandprecoder W₁ is derived at the receiving device as W₁=W_(n) ₁ ⁽¹⁾W_(k) ₁⁽²⁾, and the subband precoder W₂ is derived as W₂=W_(n) ₁ ⁽¹⁾W_(k) ₂⁽²⁾. A wideband CQI, denoted as CQI₁, represents the channel qualityassuming the wideband precoding matrix is used at the transmittingdevice. A subband CQI, denoted as CQI₂, represents the channel qualityassuming the subband precoding matrix is used at the transmittingdevice.

In one embodiment, the index k1 is not reported but it is pre-defined atboth the transmitting device and receiving device based on a commonassumption, the matrix which the index k1 represents may be theprincipal eigenvector of the matrix R, or the best approximation of theprincipal eigenvector of the matrix R by a vector in the codebook C2.

Note that for persons skilled in the art, the embodiment above may beused for at least one subband.

The method above allows for reducing the number of reported indices andsave the system resources, without adversely affecting performance.

It is understandable to those skilled in the art that all or part of thesteps of the foregoing embodiments may be implemented by hardwareinstructed by a computer program. The program may be stored in acomputer-readable storage medium. When being executed, the programperforms all or part of the steps in foregoing method embodiments.

In another embodiment of the present invention, a receiving device thatcan realize all the above methods is also disclosed. In this embodiment,the receiving device may be a user equipment.

Referring to FIG. 4, the receiving device 1900 includes:

-   -   a receiver 1901, configured to receive a reference signal from a        transmitting device;    -   a processor 1902, configured to derive a rank indication, a        codebook index n1 in a first codebook C1 and codebook indices k1        and k2 in a second codebook C2, a wideband channel quality        indicator CQI1 and a subband channel quality indicator CQI2        based on the reference signal; and    -   a transmitter 1903, configured to send the rank indication, the        codebook index n1 in the first codebook C1, the codebook indices        k1 and k2 in the second codebook C2, the wideband CQI1 and the        subband CQI2 to the transmitting device.

In addition, the receiver 1901 is further configured to receive, fromthe transmitting device, data precoded by a first precoder which isdetermined by the transmitting device based on the rank indication andthe indices n1, k1 and k2.

Note that for persons skilled in the art, the embodiment above may beused for at least one subband.

The embodiment above allows for reducing the number of reported indicesand save the system resources, without performance reduction.

In another embodiment of the present invention, a transmitting device isdescribed as in FIG. 5. In this embodiment, the transmitting device maybe a base station.

The transmitting device includes the following:

-   -   a receiver 2001, configured to receive a rank indication, a        codebook index n1 in a first codebook C1, codebook indices k1        and k2 in a second codebook C2, the wideband channel quality        indicator CQI1 and the subband channel quality indicator CQI2        from a receiving device;    -   a processor 2002, configured to derive a first precoder based on        the rank indication and the indices n1, k1 and k2, and    -   precode data to be sent to the receiving device using the first        precoder; and    -   a transmitter 2004, configured to send the precoded data to the        receiving device.

The first precoder comprises a second precoder and a third precoder,where the second precoder is a wideband precoder determined based on theindices n1 and k1, and the third precoder is a subband precoderdetermined based on the indices n1 and k2.

In one embodiment, the second precoder and the third precoder aredetermined by the following process.

The second precoder is determined by:

-   -   determining matrix W_(n) ₁ ⁽¹⁾ using the index n1 in the        codebook C1;    -   determining matrix W_(k) ₁ ⁽²⁾ using the index k1 in the        codebook C2; and    -   determining the second precoder W₁ as W₁=W_(n) ₁ ⁽¹⁾W_(k) ₁ ⁽²⁾.

The third precoder is determined by:

-   -   determining matrix W_(n) ₁ ⁽¹⁾ using the index n1 in the        codebook C1;    -   determining matrix W_(k) ₂ ⁽²⁾ using the index k2 in the        codebook C2; and    -   determining the third precoder W₂ as W₂=W_(n) ₁ ⁽¹⁾W_(k) ₂ ⁽²⁾.

In another embodiment, the second precoder and the third precoder aredetermined by the following process.

The second precoder is determined by:

-   -   determining matrix W_(k) ₁ ⁽²⁾ using the index k1 in the        codebook C2;    -   determining matrix W_(n) ₁ ⁽¹⁾ using the index n1 in the        codebook C1; and    -   determining the second precoder W₁ as W₁=W_(k) ₁ ⁽²⁾W_(n) ₁ ⁽¹⁾.

The third precoder is determined by:

-   -   determining matrix W_(k) ₂ ⁽²⁾ using the index k2 in the        codebook C2;    -   determining matrix W_(n) ₁ ⁽¹⁾ using the index n1 in the        codebook C1; and    -   determining the third precoder W₂ as W₂=W_(k) ₂ ⁽²⁾W_(n) ₁ ⁽¹⁾.        Note that for persons skilled in the art, the embodiment above        may be used for at least one subband.

The embodiment above allows for reducing the number of reported indicesand save the system resources, without adversely affecting performance.

In one embodiment, a method for precoding in a wireless communicationsystem is detailed in FIG. 6. The method include:

-   -   S2101: a transmitting device receives a rank indication, a        codebook index n1 in a first codebook C1, codebook indices k1        and k2 in a second codebook C2, the wideband channel quality        indicator CQI1 and the subband channel quality indicator CQI2        from a receiving device, wherein the receiving device may be a        user equipment;    -   S2102: the transmitting device derives a first precoder based on        the rank indication and the indices n1, k1 and k2;    -   S2103: the transmitting device precodes data using the first        precoder; and    -   S2104: the transmitting device sends the precoded data to the        receiving device.

The transmitting device derives the first precoder determined based onthe rank indication, a second precoder and a third precoder, in whichthe second precoder, which represents a wideband precoder, is determinedby the indices n1 and k1, and the third precoder, which represents asubband precoder, is determined by the indices n1 and k2.

The second precoder W₁ is determined as W₁=W_(n) ₁ ⁽¹⁾W_(k) ₁ ⁽²⁾, inwhich the W_(n) ₁ ⁽¹⁾ represents the matrix determined by the index n1in the codebook C1, and the W_(k) ₁ ⁽²⁾ represents the matrix determinedby the index k1 in the codebook C2; and

-   -   the third precoder W₂ is determined as W₂=W_(n) ₁ ⁽¹⁾W_(k) ₂        ⁽²⁾, in which the W_(n) ₁ ⁽¹⁾ represents the matrix determined        by the index n1 in the codebook C1, and the W_(k) ₂ ⁽²⁾        represents the matrix determined by the index k2 in the codebook        C2.

Alternatively, the second precoder W₁ is determined as W₁=W_(k) ₁⁽²⁾W_(n) ₁ ⁽¹⁾, in which the W_(k) ₁ ⁽²⁾ represents the matrixdetermined by the index k1 in the codebook C2, and the W_(n) ₁ ⁽¹⁾represents the matrix determined by the index n1 in the codebook C1; and

-   -   the third precoder W₂ is determined as W₂=W_(k) ₂ ⁽²⁾W_(n) ₁        ⁽¹⁾, in which the W_(k) ₂ ⁽²⁾ represents the matrix determined        by the index k2 in the codebook C2, and the W_(n) ₁ ⁽¹⁾        represents the matrix determined by the index n1 in the codebook        C1.

Note that for persons skilled in the art, the embodiment above may beused for at least one subband.

The embodiment above allows for reducing the number of reported indicesand save the system resources, without adversely affecting performance.

In one embodiment, a system for precoding is detailed in FIG. 7. Thesystem 1700 includes a transmitting device 1701 and a receiving device1702. The transmitting device 1701 may be a base station and thereceiving device 1702 may be a user equipment.

The receiving device 1702 may include a receiver, configured to receivea reference signal from the transmitting device;

-   -   a processor, configured to derive a rank indication, a codebook        index n1 in a first codebook C1 and codebook indices k1 and k2        in a second codebook C2, a wideband channel quality indicator        CQI1 and a subband channel quality indicator CQI2, based on the        reference signal;    -   a transmitter, configured to send the rank indication, the        codebook index n1 in the first codebook C1, the codebook indices        k1 and k2 in the second codebook C2, the wideband CQI1 and the        subband CQI2 derived by the deriving unit, to the transmitting        device; and    -   the receiver is further configured to receive, from the        transmitting device, data precoded by a first precoder which is        determined by the transmitting device based on the rank        indication and the indices n1, k1 and k2.

The transmitting device 1701 may include a receiver, configured toreceive a rank indication, a codebook index n1 in a first codebook C1,codebook indices k1 and k2 in a second codebook C2, a wideband channelquality indicator CQI1 and a subband channel quality indicator CQI2 fromthe receiving device;

-   -   a processor, configured to derive a first precoder based on the        rank indication and the indices n1, k1 and k2 and precode the        data to be sent to the receiving device using the first        precoder; and    -   a transmitter, configured to send the precoded data to the        receiving device.

The first precoder comprises a second precoder and a third precoder,where the second precoder is a wideband precoder determined based on theindices n1 and k1, and the third precoder is a subband precoderdetermined based on the indices n1 and k2.

In one embodiment, the second precoder and the third precoder aredetermined by the following process.

The second precoder is determined by:

-   -   determining matrix W_(n) ₁ ⁽¹⁾ using the index n1 in the        codebook C1;    -   determining matrix W_(k) ₁ ⁽²⁾ using the index k1 in the        codebook C2; and    -   determining the second precoder W₁ as W₁=W_(n) ₁ ⁽¹⁾W_(k) ₁ ⁽²⁾.

The third precoder is determined by:

-   -   determining matrix W_(n) ₁ ⁽¹⁾ using the index n1 in the        codebook C1;    -   determining matrix W_(k) ₂ ⁽²⁾ using the index k2 in the        codebook C2; and    -   determining the third precoder W₂ as W₂=W_(n) ₁ ⁽¹⁾W_(k) ₂ ⁽²⁾.

In another embodiment, the second precoder and the third precoder aredetermined by the following process.

The second precoder is determined by:

-   -   determining matrix W_(k) ₁ ⁽²⁾ using the index k1 in the        codebook C2;    -   determining matrix W_(n) ₁ ⁽¹⁾ using the index n1 in the        codebook C1; and    -   determining the second precoder W₁ as W₁=W_(k) ₁ ⁽²⁾W_(n) ₁ ⁽¹⁾.

The third precoder is determined by:

-   -   determining matrix W_(k) ₂ ⁽²⁾ using the index k2 in the        codebook C2;    -   determining matrix W_(n) ₁ ⁽¹⁾ using the index n1 in the        codebook C1; and    -   determining the third precoder W₂ as W₂=W_(k) ₂ ⁽²⁾W_(n) ₁ ⁽¹⁾.        Note that for persons skilled in the art, the embodiment above        may be used for at least one subband.

The embodiment above allows for reducing the number of reported indicesand save the system resources, without adversely affect performance.

It is envisioned that the method according to the above-mentionedembodiments may be realized by computer programs executable by theprocessors of the transmitting device and the receiving device. Thetransmitting device and the receiving device may each be equipped orcoupled with a non-transitory storage medium for storing the programs.The storage medium may be a read-only memory (ROM), a magnetic disk or acompact disk (CD).

Although the invention is described through some exemplary embodiments,the invention is not limited to such embodiments. It is apparent thatthose skilled in the art can make modifications and variations to theinvention without departing from the scope of the invention. Theinvention is intended to cover the modifications and variations providedthat they fall in the scope of protection defined by the followingclaims or their equivalent.

What is claimed is:
 1. A method for precoding data in a wirelesscommunication system, comprising: receiving, by a receiving device, atleast one reference signal from a transmitting device; deriving, by thereceiving device, a rank indication, a first codebook index in a firstcodebook, and two second codebook indices in a second codebook, based onthe at least one reference signal; sending, by the receiving device, therank indication, the first codebook index in the first codebook, and thetwo second codebook indices in the second codebook, to the transmittingdevice; and receiving, by the receiving device, data precoded by asecond precoder and a third precoder from the transmitting device,wherein the second precoder is determined based on the first codebookindex and one of the two second codebook indices, and the third precoderis determined based on the first codebook index and the other one of thetwo second codebook indices.
 2. The method according to claim 1, whereinthe second precoder is a wideband precoder and the third precoder is asubband precoder.
 3. The method according to claim 1, wherein the firstcodebook and the second codebook are configured in both the transmittingdevice and the receiving device.
 4. The method according to claim 1,wherein sending the rank indication, the first codebook index in thefirst codebook, and the two second codebook indices in the secondcodebook to the transmitting device comprises: sending, by the receivingdevice, a first message that contains the rank indication; sending, bythe receiving device, a second message that contains the first codebookindex and one of the two second codebook indices; and sending, by thereceiving device, a third message that contains the other one of the twosecond codebook indices.
 5. A receiving device, comprising: a receiver,configured to receive at least one reference signal from a transmittingdevice; a processor, configured to derive a rank indication, a firstcodebook index in a first codebook, and two second codebook indices in asecond codebook, based on the at least one reference signal; and atransmitter, configured to send the rank indication, the first codebookindex in the first codebook, and the two second codebook indices in thesecond codebook, to the transmitting device; wherein the receiver isfurther configured to receive, from the transmitting device, dataprecoded by a second precoder and a third precoder, wherein the secondprecoder is determined based on the first codebook index and one of thetwo second codebook indices, and the third precoder is determined basedon the first codebook index and the other one of the two second codebookindices.
 6. The receiving device according to claim 5, wherein thereceiving device is a user equipment in communication with a networkentity in a wireless communication system.
 7. The receiving deviceaccording to claim 6, wherein the user equipment is a mobile device, acellular telephone, a wireless enabled personal digital assistant or awireless enabled laptop computer, and the network entity is a basestation, an evolved NodeB or an access point.
 8. A method for precodingdata in a wireless communication system, comprising: receiving, by atransmitting device, a rank indication, a first codebook index in afirst codebook, and two second codebook indices in a second codebook,from a receiving device; determining, by the transmitting device, asecond precoder and a third precoder, wherein the second precoder isdetermined based on the first codebook index and one of the two secondcodebook indices, and the third precoder is determined based on thefirst codebook index and the other one of the two second codebookindices; precoding, by the transmitting device, data to be sent to thereceiving device by the second precoder and the third precoder; andsending, by the transmitting device, the precoded data to the receivingdevice.
 9. The method according to claim 8, wherein the second precoderis a wideband precoder and the third precoder is a subband precoder. 10.The method according to claim 8, wherein determining the second precodercomprises: determining a matrix W_(n) ₁ ⁽¹⁾ based on the first codebookindex in the first codebook; determining a matrix W_(k) ₁ ⁽²⁾ based onone of the two second codebook indices in the second codebook; anddetermine the second precoder W₁ based on W₁=W_(n) ₁ ⁽¹⁾W_(k) ₁ ⁽²⁾wherein n1 and k1 represent the first codebook index and the one of thetwo second codebook indices respectively; and wherein determining thethird precoder comprises: determining a matrix W_(n) ₁ ⁽¹⁾ based on thefirst codebook index in the first codebook; determining a matrix W_(k) ₂⁽²⁾ based on the other one of the two second codebook indices in thesecond codebook; and determining the third precoder W₂ based on W₂=W_(n)₁ ⁽¹⁾W_(k) ₁ ⁽²⁾ wherein n1 and k2 represent the first codebook indexand the other one of the two second codebook indices respectively. 11.The method according to claim 8, wherein determining the second precodercomprises: determining a matrix W_(n) ₁ ⁽¹⁾ based on the first codebookindex in the first codebook; determining a matrix W_(k) ₁ ⁽²⁾ based onthe one of the two second codebook indices in the second codebook; anddetermine the second precoder W₁ based on W₁=W_(k) ₁ ⁽²⁾W_(n) ₁ ⁽¹⁾,wherein n1 and k1 represent the first codebook index and the one of thetwo second codebook indices, respectively; and wherein determining thethird precoder comprises: determining a matrix W_(n) ₁ ⁽¹⁾ based on thefirst codebook index in the first codebook; determining a matrix W_(k) ₂⁽²⁾ based on the other one of the two second codebook indices in thesecond codebook; and determining the third precoder W₂ based on W₂=W_(k)₂ ⁽²⁾W_(n) ₁ ⁽¹⁾, wherein n1 and k2 represent the first codebook indexand the other one of the two second codebook indices respectively. 12.The method according to claim 8, wherein the first codebook and thesecond codebook are configured in both the transmitting device and thereceiving device.
 13. A transmitting device, comprising: a receiver,configured to receive a rank indication, a first codebook index in afirst codebook, and two second codebook indices in a second codebook,from a receiving device; a processor, configured to determine a secondprecoder and a third precoder based on the rank indication and the firstcodebook index and the two second codebook indices, and precode data tobe sent to the receiving device by the second precoder and the thirdprecoder; and a transmitter, configured to send the precoded data to thereceiving device; wherein the second precoder is determined based on thefirst codebook index and one of the two second codebook indices, and thethird precoder is determined based on the first codebook index and theother one of the two second codebook indices.
 14. The transmittingdevice according to claim 13, wherein the second precoder is a widebandprecoder and the third precoder is a subband precoder.
 15. Thetransmitting device according to claim 13, wherein the transmittingdevice is a network entity in communication with a user equipment in awireless communication system.
 16. The transmitting device according toclaim 15, wherein the user equipment is a mobile device, a cellulartelephone, a wireless enabled personal digital assistant or a wirelessenabled laptop computer, and the network entity is a base station, anevolved NodeB or an access point.